1. Field of the Invention
This invention relates generally to implantable medical devices, and more particularly to medical devices that may be activated or adapted to one or more physiological conditions in a patient in which the devices are implanted.
2. Background
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, including epilepsy and other motor disorders, and conditions caused by or involving abnormal neural discharge. One of the more recently available treatments involves applying an electrical signal to reduce symptoms or effects of such neural disorders. For example, electrical signals have been successfully applied to neural tissue in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions such as depression. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. No. 4,702,254 to Dr. Jacob Zabara, which is hereby incorporated by reference in its entirety in this specification.
Electrical stimulation of the vagus nerve may be provided by implanting an electrical device underneath the skin of a patient, detecting a precursor, symptom or effect associated with the condition, and delivering electrical stimulation pulses to the vagus nerve. Alternatively, the system may operate without a detection system if the patient has been diagnosed with epilepsy, and the device may simply apply a series of electrical pulses to the vagus nerve (or another cranial nerve) intermittently throughout the day, or over another predetermined time interval. Stimulation that involves a detection and/or sensing operation is referred to as active stimulation, while stimulation without a detection or sensing operation is known as passive stimulation.
Many implantable pulse generators used for electrical stimulation of neurological tissue operate according to a therapy algorithm programmed into the device by a health care provider such as a physician. One or more parameters of the therapy (e.g., current amplitude, pulse width, pulse frequency, and on-time and off-time) may thereafter be changed by reprogramming the neurostimulator after implantation by transcutaneous communication between an external programming device and the implanted neurostimulator. The ability to program (and later re-program) the implanted device permits a health care provider to customize the stimulation therapy to the patient's needs, and to update the therapy periodically should those needs change.
It is desirable, however, for an implantable medical device, such as a neurostimulator, to be able to provide active stimulation by automatically detecting one or more physiological parameters and responsively initiating a stimulation therapy specifically tailored to the physiological parameters detected, without the necessity of intervention by a health care provider. The detected parameters preferably indicate the onset or potential onset of an undesirable physiological event, such as an epileptic seizure. Detection of such physiological events is, however, complicated by physiological differences among patients.